|Publication number||US6965625 B2|
|Application number||US 09/961,707|
|Publication date||Nov 15, 2005|
|Filing date||Sep 24, 2001|
|Priority date||Sep 22, 2000|
|Also published as||EP1342299A2, US20020097755, WO2002025785A2, WO2002025785A3, WO2002025785A8|
|Publication number||09961707, 961707, US 6965625 B2, US 6965625B2, US-B2-6965625, US6965625 B2, US6965625B2|
|Inventors||Michael R. Mross, Thomas H. Lowell, Robert A. Durant, Peter B. Sewell|
|Original Assignee||Vermont Photonics, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (31), Non-Patent Citations (29), Referenced by (38), Classifications (13), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application claims priority to U.S. Provisional Patent Application No. 60/235,149, filed 22 Sep. 2000, the contents of which are herein incorporated by reference in their entirety.
This invention relates generally to apparatus and methods for generating coherent electromagnetic radiation using free electron lasers (FEL).
In the late 1980's, the research of the late Professor John Walsh at Dartmouth College and others led to the development of tunable, electron beam driven radiation sources to produce electromagnetic radiation at terahertz frequencies in a flexible, tunable, and affordable fashion. Walsh's tunable terahertz lasers are fully described in U.S. Pat. Nos. 5,263,043 and 5,790,585 (hereinafter referred to as “the Walsh patents”). Walsh's work showed that a small, compact, and relatively inexpensive table top free electron laser which utilizes a diffraction grating element could be a commercially practical device to generate far infrared (FIR) or terahertz electromagnetic waves.
The Walsh patents essentially disclose FEL apparatuses that pass an electron beam over a surface of a diffraction grating element (i.e., a resonator) thereby producing electromagnetic radiation. The diffraction grating elements of the Walsh patents are fabricated by machining slots in an aluminum block. With this manufacturing approach, the minimum width of the slots largely depends upon the width of the saw blade used. This can limit the slot widths that can be fabricated. Additionally, the choice of block materials may also be limited since some materials cannot be cleanly machined without chipping or burring occurring at the edges (e.g., in the case of hard materials). Others may tend to flow when machined (e.g., in the case of softer materials). Availability of grating slot dimensions may also be limited as saw blades cannot be made to be arbitrarily thin. Furthermore, the depth and the shape of the interior profile of the slots and the shape of the external surfaces between the slots are not easily controlled in these manufacturing techniques. The saw blade, in its cutting action, may leave burrs on exterior comers and rounded surfaces on interior comers. According to the theory developed by Walsh, the width, depth and shape of the slots is critical in determining the wavelength of operation and the strength of the coupling between the bound electromagnetic radiation modes and the electron beam.
The present disclosure is directed to laser apparatuses for generating coherent electromagnetic laser radiation having an electron beam generator for generating a beam of electrons, a diffraction grating element, wherein the beam of electrons is directed over the diffraction grating element, and having at least one wing element coupled to the diffraction grating element. In some embodiments, the wing element(s) can be coupled to a top portion of the diffraction grating element. While in others, the wing element(s) can be coupled to a side portion of the diffraction grating element. Additionally, the wing element(s) may have at least one surface that is polished.
In accordance with one aspect of the present disclosure, the wing element(s) can be comprised of a conductive metal, such as copper or aluminum, for example.
In accordance with another aspect of the disclosure, the at least one wing element can be substantially planar, concave or convex. Many embodiments in accordance with the disclosure comprise a set of wing elements coupled to a diffraction grating element. In such embodiments, the set of wing elements may be symmetrical and/or substantially parallel to each other; the set of wing elements may also diverge away from each other or alternatively, converge towards each other.
In accordance with a further aspect of the present disclosure, certain embodiments of the laser apparatus can include a cooling system for cooling the diffraction grating element. In other embodiments, the cooling system can be used to cool the at least one wing element. In yet other embodiments, the cooling system can be used to cool the diffraction grating element and the at least one wing element.
In accordance with yet a further aspect of the present disclosure, the diffraction grating element and the wing element(s) can be manufactured as a micromachine monolithic structure.
The present disclosure is also directed to methods of manufacturing diffraction grating elements comprising obtaining at least one primary conductive sheet and at least one secondary conducting sheet, wherein the height of the at least one primary conductive sheet is different from the height of the at least one secondary conductive sheet, placing the at least one secondary conducting sheet on the at least one primary conductive sheet to form a stack of primary and secondary conductive sheets, and then securing the stack of primary and secondary conductive sheets together. In some embodiments, the stack of primary and secondary conductive sheets can consist of alternating sheets of primary and secondary conductive sheets. In yet other embodiments, the thickness of the at least one primary conductive sheet may be different from the thickness of the at least one secondary conductive sheet. Additionally, the stack of primary and secondary conductive sheets may be secured via a clamping device.
In accordance with another aspect of the disclosure, the methods of manufacturing diffraction grating elements can further consist of placing the at least one primary conductive sheet in a tool fixture before placing the at least one a secondary conducting sheet on the at least one primary conductive sheet, and then aligning the at least one primary conductive sheet and the at least one secondary conducting sheet in the tool fixture. In certain embodiments, the tool fixture can serve to secure the stack of primary and secondary conductive sheets.
In accordance with yet a further aspect of the present disclosure, the at least one primary conductive sheet and the at least one secondary conducting sheet can be comprised of copper. Alternatively, the at least one primary conductive sheet and the at least one secondary conducting sheet can be comprised of aluminum.
Further features and advantages of the present invention will become apparent from the following description of the preferred embodiment and from the claims.
This disclosure is directed to grating coupling free electron laser apparatuses for generating coherent electromagnetic laser radiation and methods for fabricating the same. Grating coupling free electron laser apparatuses are described in U.S. Pat. Nos. 5,263,043 and 5,790,585, both issued to Walsh, the contents of which are incorporated herein by reference in their entireties.
There is a need to develop methods for fabricating diffraction grating elements which provide greater control over the over the gratings' slot width, slot depth and shape. Additionally, since the diffraction grating element is so vital to the production of the desired coherent electromagnetic laser radiation, there is a need to develop improvements over the diffraction grating elements disclosed in the Walsh patents.
Laser apparatuses for generating coherent electromagnetic laser radiation, as applicable herein, comprise an electron beam generator and a diffraction grating element. The electron beam generator generates a beam of electrons that can be directed over the diffraction grating element to produce coherent electromagnetic laser radiation with a desired frequency. For the apparatuses and methods described herein, an electron gun from a disassembled AMRAY 1000 scanning electron microscope was used as the electron beam generator. The electron gun comprises a Spellman High Voltage Power Supply SL50N300/AOL/LR having a max output 50KV at 6ma, however, a wide variety of electron gun and power supply combinations can be used as an electron beam generator. Persons skilled in the art will readily recognize the various embodiments that the electron beam generator can take.
In an attempt to enhance performance, and more specifically, to increase the power (i.e., amplitude) of the generated laser radiation, wing elements were coupled to the diffraction grating element, as depicted in
In greater detail, these Figures illustrate different sets of wing elements having different shapes coupled to the diffraction grating element using variously different wing separation dimensions (i.e., the “gap” distance between a first wing element and a second wing element), wing lengths and widths. The introduction of the wing elements can improve power output from 20% to 300% as compared to the power output levels produced from the same diffraction grating elements having no wing elements. A wide variety of conductor that can be fabricated, and preferably polished on one surface (i.e., the surface facing the grating), can be used as a wing element. Also, higher conductivity of the materials used in the wing element can result in improved performance.
It is surmised that since the electron beam used herein generally has a diameter of approximately 25 to 50 microns where it interacts with the grating element that the remainder of the grating width (˜6000 microns) might interfere with the generation of Thz radiation by absorbing energy that might otherwise be useful to the generation of Thz. Furthermore, the inclusion of wing elements appears to improve the output coupling of the Thz radiation with free space as well as enhance electron beam interaction with the diffraction grating element.
Wing elements may be coupled to the diffraction grating element and aligned (e.g., substantially parallel) to the electron beam at various distances apart (i.e., the gap) along the full length of the grating. The wing elements protrude out from the face (i.e., the top surface) of the grating element for some distance, such as 6 mm, for example. The wing elements thus can be to cover the “unused” part of the grating element. The wing elements may be used with diffraction grating elements having different geometries (e.g., different d, p and s, etc.).
Experimentally, the addition of wing elements yielded the following results:
With no wing elements
17 mV detected at the detector
With wing elements,
32 mV detected at the detector
gap set at 3.5 mm
With no wing elements
21 mV detected at the detector
With wing elements,
72 mV detected at the detector
gap set at 1.6 mm
With wing elements,
103 mV detected at the detector
gap set at 1.0 mm
These results illustrate that the Thz power outputs improved (i.e., increased) when the wing elements were used, and that further increases in Thz power resulted when the gap between the two wing elements was decreased. Thus, the combination us using wing elements with a narrow grating element having an effective width (i.e., the width of the grating element which is exposed to a electron beam) of up to fifty times the electron beam diameter appears to increase the energy output coupling. Another indication of the effectiveness of the wing elements and/or the effectiveness of the narrowing of the grating element is that the grating element “turns on” more easily, meaning at a lower electron beam current for a given voltage.
An exemplary embodiment of wing elements 20 a and 20 b coupled to a diffraction grating element 10 is shown in
The wing elements 20 a and 20 b and the diffraction grating element 10 may be comprised of materials that are electrically conductive, such as conductive metals, for example. Due to the conductive (and thermal) properties of copper, the wing elements 20 a and 20 b and diffraction grating element 10 preferably are made of copper, or alternatively, at a minimum, the diffraction grating element 10 and surfaces 21 a and 21 b of wing elements 20 a and 20 b, respectively, preferably are comprised of copper. As an alternative to copper, aluminum or another conductive material may also be used. Moreover, in preferred embodiments, wing element 20 a (i.e., a first wing element) and wing element 20 b (i.e., a second wing element) are symmetrical—meaning that wing element 20 a and wing element 20 b are mirror-images of each other.
Moreover, the set of wing elements 1-5 (or 20 a and 20 b) can be coupled to the diffraction grating element so that the wing elements are aligned (i.e., oriented) parallel to each other. For example, in the set of concave wing elements 3, wing elements 3 a and 3 b, can be coupled to the diffraction grating element 10 so that the orientation of wing element 3 a (e.g., in relationship to the electron beam axis of travel 300) is substantially parallel to the orientation of wing element 3 b.
Similar to the “expanding” diffraction grating element 10 described above, the diffraction grating element 10 can also be made to be “contracting” wherein the gap found between a pair of wing elements, 20 a and 20 b, for example, decrease as one travels along the electron beam axis of travel 300. This can be accomplished in much the same manner as discussed above.
Furthermore, the diffraction grating element 10 and wing elements 20 a and 20 b (or 1-6) could be manufacturing as a micromachine monolithic structure by using semiconductor chip manufacturing technologies known in the art. For example, PCT patent application WO 00/72413 A2, the contents of which are herein incorporated by reference in their entirety, describes such methods in regards to manufacturing diffraction grating structures. Such manufacturing methodologies can be used such that the resultant diffraction grating element 10 and wing elements 20 a and 20 b structures are electrically conductive.
Diffraction Grating Element Fabrication
Fabricating the diffraction grating element 10 may be done in a variety of ways. Different methods of these permit the manufacturing of such diffraction grating element 10 out of a wide variety of conducting materials.
Because of its high conductivity and availability in sheets of uniform thickness, copper sheets may be chosen for the primary conductive sheets 61 and secondary conducting sheets 62. For example, the primary conductive sheets 61 may be formed from 0.005 inch copper sheet (e.g., readily-available copper shims). The secondary conducting sheets 62 may be formed from 0.001 inch copper sheet, with a difference in height of 0.004 inch (between the height of the primary conducting sheets 61 and the secondary conductive sheets 62). The diffraction grating element can feature alternating sheets of primary conductive sheets 61 and secondary conductive sheets 62. In addition to copper, a wide variety of electrical conductive sheet material, including aluminum, for example, can be used.
In experiments, the diffraction grating element 10 manufactured from the primary conductive sheets 61 and secondary conductive sheets 62 produced almost twice the output power at wavelengths around 500 μm as a similar-sized grating element produced by the techniques of the Walsh patents. The improvement in power output can be attributed to a combination of:
One potential advantage of using these methods of manufacturing a diffraction grating element 10 is that commercially-available sheet stock (e.g., copper sheet stock) may be well suited to be used as primary conductive sheets 61 and secondary conducting sheets 62, depending upon the geometry of the diffraction grating element 10 desired. If the available sheet stock is not well suited to the desired geometry, then sheet stock may be rolled to achieve a thickness that will lend itself to the desired geometry of a diffraction grating element. Similarly, available sheet stock may sheared to obtain the primary conductive sheets' 61 height h(p) and/or secondary conducting sheets' 62 h(s) that are better suited to the geometry of a desired diffraction grating element 10.
The stack of primary conductive sheets 61 and secondary conductive sheets 62 can be secured via mechanical fasteners (screws, rivets, etc.), mechanical clamping devices or by bonding methods using solder, welding, brazing, epoxies, glues, bonding agents and the like.
Cooling of the Diffraction Grating Element and/or Wing Elements
In grating-coupling free electron lasers, the resonator structure surfaces which come into contact with the electromagnetic fields are electrically conductive. The conductivity of these surfaces plays a role in the efficiency of the resonator structure. Lower conductivity generally means higher losses of energy of the electromagnetic field and therefore a lower “Q” of the resonator, and higher conductivity means lower losses and higher “Q”.
Since the electrical conductivity of most metals increase as the temperature of the metal decreases, the output power of grating-coupling free electron lasers will increase as the temperatures of the resonator structures of the lasers are lowered. Many metals can be cryogenically cooled (e.g. to liquid nitrogen temperature or even to liquid Helium temperature) to increase the conductivity of the metal; some metals can even be made to be superconducting by lowering the temperature of the metal to its critical temperature. For example, the conductivity of copper at liquid nitrogen temperature is substantially higher then the conductivity of copper at room temperature.
The present invention has been disclosed in connection with various laser apparatuses and methods for fabricating diffraction grating elements as shown and described. Various modifications and improvements thereon will, however, become readily apparent to those skilled in the art. The description of the laser apparatuses and methods for fabricating diffraction grating elements should therefore be considered only as illustrative, and not as limiting, of the present invention. The spirit and scope of the present invention is to be limited only by the following claims.
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|WO2007130095A3 *||Jun 12, 2006||Jan 17, 2008||Virgin Islands Microsystems||Single layer construction for ultra small devices|
|U.S. Classification||372/74, 372/2, 372/102|
|International Classification||H01S1/00, H01S3/30, H01S3/04, H01S3/09, H01S3/08|
|Cooperative Classification||H01S3/0903, H01S3/08009, H01S3/0401, H01S1/00|
|Jan 14, 2002||AS||Assignment|
Owner name: VERMONT PHOTONICS, INC., VERMONT
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:MROSS, MICHAEL R.;LOWELL, THOMAS H.;DURANT, ROBERT A.;AND OTHERS;REEL/FRAME:012479/0864;SIGNING DATES FROM 20011105 TO 20011210
|Sep 22, 2005||AS||Assignment|
Owner name: VERMONT PHOTONICS TECHNOLOGIES CORP., VERMONT
Free format text: CHANGE OF NAME;ASSIGNOR:VERMONT PHOTONICS, INC.;REEL/FRAME:016571/0130
Effective date: 20031229
|Apr 15, 2009||FPAY||Fee payment|
Year of fee payment: 4
|Jun 28, 2013||REMI||Maintenance fee reminder mailed|
|Nov 15, 2013||LAPS||Lapse for failure to pay maintenance fees|
|Jan 7, 2014||FP||Expired due to failure to pay maintenance fee|
Effective date: 20131115